Re: UEFI/"BIOS" booting, was Re: USB Install Fails, Complains about CD-ROM

[David Wright <deblis@lionunicorn.co.uk>]

On Mon 14 May 2018 at 11:56:11 (-0400), Stefan Monnier wrote:
> > Yes, documentation of firmware is almost unknown in my experience
> > (since probably 30 years ago). That's why I took the least invasive
> 
> It's documented to the extent that it says "implements UEFI" and that
> UEFI is documented.

I can only speak for machines that I have access to. The earliest PCs
I remember came with booklets that had illustrations of the various
CMOS screens. Since then, I've usually had to transcribe the settings
manually into files that I archive (in case of battery failure etc.)

In this particular instance (the Lenovo), its PDF says:

  How can I change the boot mode?
    There are two boot modes: UEFI and Legacy Support. To change the boot
    mode, start the BIOS setup utility and set boot mode to UEFI or Legacy
    support on the boot menu.

and that's it, apart from implying that the Legacy mode is useful:

  When do I need to change the boot mode?
    The default boot mode for your computer is UEFI mode. If you need to
    install a legacy operating system, such as Windows (that is, any operating
    system before Windows 8), Linux or DOS, etc on your computer, you must
    change the boot mode to Legacy support. The legacy operating system,
    such as Windows, Linux or DOS, etc cannot be installed if you don't
    change the boot mode.

> >> Same here (basically for the same reason: the behavior of the firmware
> >> and OS when faced with a disk that has both a GPT and an MBR partitions
> >> is largely unspecified and will vary depending on your system).
> > Eh? I've yet to see a GPT disk that didn't have a protective MBR.
> 
> Exactly: what happens when a GPT disk has a real MBR (rather than a
> protective MBR) is "you're on your own".

As Thomas has just pointed out, there's no such thing. I didn't try
to read anything into your non-idiomatic use of the plural in "a disk
that has both a GPT and an MBR partitions". There will be any number
of partitions in the GPT, and just one in the MBR's partition table.

AIUI what's unspecified is what you choose to put in the code section
of the MBR. In my case, and probably for most people here, it will be
Stage 1 of Grub. And if one has any sense, that will be pointing to a
BIOS boot partition, which is one of the first things I set up on a
GPT disk so that I can use it in my old BIOS machines. It makes things
far more bullet-proof if Grub knows there's a safe place to put its
later Stages. I suspect that systems without ones are likely to be the
ones causes much of the reported trouble.

Where's the author's discussion of all this? My beef is that these
aspects are obfuscated with talk of magic code, magic space, magic
locations and "special sauce". That's the kind of stuff I'm placing in
*their* garbage, even if it doesn't make up 95%.

> >> It's easy to reconcile: he doesn't say your setup is impossible or can't
> >> work, he just recommends not to do that because you may encounter
> >> unexpected difficulties.  E.g. in theory an upgrade to your firmware or
> >> to one of your OSes could break it, tho in practice you're probably OK
> >> at least until you move that setup to another machine with
> >> a different firmware.
> > Not sure what you mean here. It's a laptop: nowt's going nowhere.
> 
> Take the disk out, put it in another machine (laptop or desktop, it
> doesn't matter).

At that point, I would assume windows is dead because it's not
licenced for its new home. At which point, a decision could be made
on the basis of whether the new machine could boot Grub through the
MBR. If that's not the case, then obviously one is going to have to
boot with UEFI from, say, a stick and grub-install in UEFI mode.

If that change is made correctly, the disk should be bootable in its new
location. In addition, if it were moved back to its original location,
it should now be possible to boot both windows and linux using EUFI mode.

But these are the issues I have been keen to avoid (successfully)
by booting in different modes for the two OSes. As I stated, there
was to be no disturbance of the windows system.

Cheers,
David.